Hydrocarbon exploration and development activities rely on information derived from sensors which capture data relating to the geological properties of an area under exploration. One approach used to acquire this data is through wireline logging. Wireline logging is typically performed in a wellbore immediately after a new section of hole has been drilled. These wellbores are drilled to a target depth covering a zone of interest, typically between 1000-5000 meters deep. A sensor package, also known as a “logging tool” or “tool-string” is then lowered into the wellbore and descends under gravity to the target depth of the wellbore well. The logging tool is lowered on a wireline—being a collection of electrical communication wires which are sheathed in a steel cable connected to the logging tool. Once the logging tool reaches the target depth it is then drawn back up through the wellbore at a controlled rate of ascent, with the sensors in the logging tool operating to generate and capture geological data.
There is a wide range of logging tools which are designed to measure various physical properties of the rocks and fluids contained within the rocks. The logging tools include transducers and sensors to measure properties such as electrical resistance, gamma-ray density, speed of sound and so forth. The individual logging tools are often combinable and are typically connected together to form a logging tool-string. These instruments are relatively specialised sensors, which in some cases need to be electrically isolated or located remote from metallic objects which are a source of noise in the data generated. Some sensors are designed to make close contact with the borehole wall during data acquisition whilst others are ideally centred in the wellbore for optimal results. These requirements need to be accommodated with any device that is attached to the tool-string.
The drilling of wells and the wireline logging operation is an expensive undertaking. This is primarily due to the capital costs of the drilling equipment and the specialised nature of the wireline logging systems. It is important for these activities to be undertaken and completed as promptly as possible to minimise these costs. Delays in deploying a wireline logging tool are to be avoided wherever possible.
One cause of such delays is the difficulties in lowering wireline logging tools down to the target depth of the wellbore. As the logging tool is lowered by cable down the wellbore by gravity alone, an operator at the top of the well has very little control of the descent of the logging tool. Logging tools can become held up on ledges of rock formed inside a well—usually found at the boundaries of hard rock layers where the adjacent rock layer crumbles. An operator may not immediately identify that a logging tool has become stuck on a ledge, and may also spend a significant amount of time reeling the cable and tool-string back in and attempting to move it past the obstruction formed by a ledge.
The chances of a wireline logging tools getting stuck or being impeded is also significantly increased with deviated wells. Deviated wells do not run straight vertically downwards and instead extend downward at an angle. Multiple deviated wells are usually drilled from a single surface location to allow a large area of interest to be explored. As wireline logging tools are run down a wellbore with a cable under the action of gravity, the tool-string will traverse the low side or bottom of the wellbore wall and immediately encounter any obstructions on the wellbore wall as it travels downwards to the target depth.
Furthermore, in deviated wells there is the potential for drilling cuttings to collect on the low side of the wellbore. Rock cuttings are more difficult to remove when the wellbore is deviated. The logging tool has to travel over or through these drilling cuttings, which can impede its progress and also collect in front of the logging tool. In some cases the logging tool may not be able to plough through the cuttings to reach the bottom of the wellbore.
Furthermore, as hole deviation increases, the sliding friction can prevent the logging tool descending. The practical limit is around 50-60° from the vertical, and in these high angle wells any device that can reduce friction is very valuable. The running of the tool-string over the low side surface of the borehole also needs to be taken into account in the design of the tool-string, and in particular the housing and the containment of its sensitive sensors and transducers.
Attempts have been made to address these issues in the deployment of wireline logging tools, as disclosed in U.S. Pat. Nos. 7,866,384, 7,395,881 and US patent application 20120061098. These patent specifications describe a number of different forms of an in-line roller devices integrated into the logging tool-string. These devices aim to reduce the friction experienced by a tool-string as it is run along the low side slope of a deviated wellbore.
This in-line positioning of roller devices increases the potential to cause damage to the logging tools as there are additional O-rings connections required that could potentially leak.
Furthermore, there are multiple additional electrical connections that need to be made between the individual logging tools making up the logging tool-string. The additional components and tool-string length means it takes longer to connect and disconnect the tool-string, which slows down the wireline logging operation and therefore increases well costs. Further, there is a lack of flexibility inherent in this approach as in-line roller devices can only be placed between logging tools, and as some of these tools are quite long, in-line rollers may not keep the entire tool body off the borehole wall.
The rollers employed in these forms of prior art devices also have relatively small wheels with a minimal clearances. In deviated wellbores well drilling cuttings will collect on the low side of the well, and these small sets of rollers can struggle to make headway through piles of cuttings. In situations where large amounts of cuttings are encountered these small rollers can be of no assistance at all and simply add to the length and weight of an already unwieldy tool.
McNay U.S. Pat. No. 8,011,429 and Schlumberger patent application US2013248208 describe roller assemblies which slip over the logging tool, and are mounted such that they are free to rotate about the longitudinal axis of the tool. These devices have relatively small wheels which do not rotate easily over rough surfaces. In addition, it is often the central or side part of the wheel which is in contact with the wellbore wall, rather than the circumferential or radially extreme edge. This means that the wheels are often skidding rather than rotating. Neither of these devices has an active lubrication system which can prevent contaminants from entering and jamming the bearings.
All of these existing prior art devices attempt to assist a tool-string in travelling down a deviated wellbore. However, the devices do not assist in maintaining a known orientation of the tool-string, or a specific clearance or “standoff” between the active part of the tool-string and the wellbore wall.
U.S. Pat. No. 684,732 describes a sectional threading rod which is provided with wheels to reduce friction as the rod is pushed down a conduit. The wheels have an axle which is above the centre of the rod. However, the rod is not intended for use with a logging tool.
Other attempts to address the issues associated with deviated wellbores include a number of prior art “hole finding” devices. For example U.S. Pat. No. 4,474,235, US patent application US 20120061098, PCT publication WO 2010/106312 and US patent application US20120222857 all describe systems for wireline hole finding devices which rely on one or more rollers located at the nose at the bottom of a tool-string. The nose is the leading end at the bottom of the tool-string during descent of the wellbore. These rollers are arranged to allow the nose of the tool-string to roll into, and then up and over, irregularities and obstructions in a wellbore.
However due to the use of a number of metallic components these types of hole finding devices are not necessarily compatible with induction type resistivity tools which are generally the most commonly used sensor in such applications. Also, some logging tools are “bottom only”, and have sensors which must be located at the lower extremity of the tool-string. The prior art hole finders are heavy and metallic and hence not compatible with such tools.
Furthermore, these systems are relatively complicated and must be appropriately designed and maintained to withstand the hostile environment experienced at depth in exploration wells. If the moving parts used in these systems cease to function the hole finder is ineffective. These designs are also relatively heavy and inflexible. Any impact forces or torque acting on the hole finder are transmitted into the tool-string, potentially causing damage to the sensors.
Another approach used in the design of hole finding devices is disclosed in US patent application US 20090145596. This patent specification describes an alternative hole finding system employed outside of wireline applications where a conduit, tubing or pipe is attached to the sensor tool in order to push it down the wellbore. This specification discloses a relatively complicated system which requires a surface operator to actively adjust the orientation of a nose assembly mounted at the bottom of the tool. The specification also discloses that this device requires a range of sensors that are used to detect sensor tool movement, and specifically if the sensor tool is held up. This form of hole finding system is again relatively heavy and complex. Furthermore, a dedicated operator is also required to monitor the progress of the sensor tool to actively adjust the orientation and angle of attack of the adjustable nose assembly when the sensors detect that the sensor device is held up as it moves down the wellbore. A similar design is described in U.S. Pat. No. 7,757,782. This device is also an active system which requires manipulation from an operator at the surface to change the nose angle and azimuth in order in order to navigate past obstacles after the logging tool is obstructed.
It would be an advantage to have an improved logging tool assembly comprising sensor transportation apparatuses to carry the assembly down a wellbore which addressed any or all of the above issues or at least provided an alternative choice. An improved logging tool assembly with sensor transportation apparatuses and orientation structure which could also be used to orient the logging tool assembly would also be of advantage.
The reference to any prior art in the specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge in any country.